The present invention relates generally to an ignition waveform analyzer and more particularly to an apparatus for measuring the dwell or dwell ratio of an automotive ignition system.
In an internal combustion engine, the spark plugs are fired by alternately connecting and disconnecting one end of the primary ignition coil and ground. The switching device, in a conventional ignition system, is an electromechanical circuit breaker or switch, commonly referred to as the "points". In an electronic ignition system, switching is performed by a solid state device, controlled by the points. The solid state switch, generally a power transistor, operates in a partial conduction mode to enhance engine performance.
As a result of this partial conduction capability, the primary coil voltage in an electronic ignition system varies from zero volts during the dwell period, in contrast to the conventional ignition system. In the presently known automotive test euipment, however, measurement of the dwell angle depends upon a zero or near zero volt primary signal. Thus, the presently available equipment is often incapable of accurately measuring the dwell angle of a solid state ignition system.
Various attempts have been made to design a voltage discriminator, which classifies primary coil voltages within a predetermined zone or window as "zero volts". Unfortunately, problems are immediately encountered. If the discrimination level is too low, then the dwell time, especially at low speeds, is inaccurately short, as the primary coil voltage increases towards the end of the dwell section. Conversely, if the discrimination level is too high, then the ringing portion of the primary waveform, particularly at high speeds, will interfere with proper dwell measurements.